Operation of alternating current radioreceivers



Oct. 16, 1934. E J. H U S 1,977,469

OPERATION OF ALTERNATING CURRENT RADIORECEIVERS Filed July 15. 1951 INF!!! 5g 1% VOL T/IGE T0 REM/UNDER CF55 I N VEN TOR.

a. M/ZWM QZZaU y w ATTORNEYS the necessary field flux for the speaker operation,

also through the elimination of expensive audio Patented Oct. 16, 1934 UNITED STATES OPERATION OF ALTERNATING CURRENT BADIORECEIVERS Emmery J. H. Bussard, Cincinnati, Ohio, assignor to The Crosley Radio Corporation, Cincinnati, Ohio, a corporation of Ohio Application July 13, 1931, ScriaI NO. 550,327

7 Claims.

In radio apparatus operated from an' alternating source of electrical current, to which my invention is applied, it has been the practice to provide a rectifying device, and then toattempt to smooth out the curve of the resultant pulsating direct current through the use of a filter system, usually comprising, a network of large capacities and inductances. Such filter systems, while they may be made entirely adequate for the purpose, are yet not only very expensive but quite bulky. Thus they have militated against portability in small A. C, sets, and their relative expense has been so great as to cause the trade to endeavor to strike a compromise, in the less expensive sets between objectionable hum and unobjectionable hum. i

The primary object of my invention is the provision'of new and improved means for eliminating the undesirable. notes or audible frequencies which are reproduced by thespeakersin A. C. driven systems, which reproduction will be understood as due to pulsating current supplied the plates of the amplifier and detector tubes from the rectifier aforesaid, and in many cases to the raw alternating current supplied to the filaments of such tubes. It is a primary object of my invention to accomplish the eliminationof hum Without resort to the expensive filter systems generally employed. It is my object to eliminate hum -with a minimum of filtering equipment, and largely by counterbalancing those forces which produce audible frequencies in the speaker, which frequencies are not the result of the desired impressed signal. Thus it is my object to achieve a quiet receiver by causing the pulsations of the operating current to neutralize each other in respect to the acoustic reproducer;

In the preferred embodiment of my invention, the exciting coil, i. e. the field winding, of a dynamic speaker may be utilized both for supplying and forsupplying the necessary grid bias voltage of the output tube or tubes, while it serves also as a unit in the filtering system of the receiver.

This will be found to be extremely advantageous since the total rectified voltage used need be only the'sum of the maximum plate voltage of the output tube and the bias voltage thereof. This permits a very considerable saving in apparatus, not only in the power transformer mechanism but filter chokes and in the filtering condensers, for reasons heretofore made apparent, and also because of the lowered voltage at which such devices may be operated in my system.

These and other objects of my invention which will beset'forth hereinafter, or will be apparent to oneskilled in the art upon reading these specifications, I accomplishby that certain construction and arrangement of parts and in-that process of which I shall now describe a preferred embodiment and a modification thereof, it being understood that my invention is applicable to radio receiving apparatus having a wide variety of circuits, and that the embodiments hereinafter described in detail are. exemplary and not limiting. Reference is now made to the accompanying drawing, wherein I V I Figure 1 shows my inventionapplied to a radio receiving set of otherwise substantially standard design; f

Figure 2 shows amodification ofv my structure which may be applied to a number of different receiving sets, and in which I have not illustrated the parts comprising the radio, audio, and detector stages of the set.

It will be understood that the drawing is largely diagrammatic.

In Figure 1, I haveshown my invention as appliedto a receiver in which I have indicated generally at 1 a radio frequency stage or amplifier consisting of one or a plurality of tubes and the circuits necessary for their operation, as will be readily understood by one skilled in the art. I have indicated a detector broadly at 2, and an audio frequency amplifier broadly at 3. I have, by way of example, shown the radio and detector tubes as being of the heater type, the filament of which may be excited by alternating current supplied through the secondary of a transformer T, or other equivalent source of supply. The audio frequency tube or tubes I have indicated as of the three element type, the filament of which is also excited directly by an alternating current.

I have shown a loud speaker in connection with which there is a transformer 5 which feeds the voice coil 5a connected to a reproducing device, such as a cone. I have also indicated diagrammatically an excitation Winding 6 for the speaker, which is a winding formed directly upon the field core structure thereof. I have illustrated these several parts in a circuit diagram without attempting to indicate their physical association in the structure of a loud speaker. It will be understood, of course, that the inductance 6, which is also the excitation winding of the loud speaker, is physically associated in a dynamic speaker construction with a suitable field core mounting,'the dynamic cone and the voice coil winding 5a. These several parts are supplied by voltage from a rectifying device 4 which I have shown as a full wave rectifying tube. It will be noted that the field excitation winding 6 is shunted directly across the output of the rectifier.

The several other parts necessary to the understanding of my invention have been shown and indicated in the figures as follows: -7- is a back voltage supply condenser employed in connection with the rectifier. denser. 9 is a resistor employed to limit the B voltage of the detector and the B voltage of the radio frequency tubes. by-pass condenser. 11 is a condenser employed to tune a section of the excitation winding-in the a figures, which figures make the circuit connec-' tions entirely clear.

In operation, a rectified voltage is supplied by the tube 4 or other. rectifying device. The current will be direct but pulsating, and will have a frequency equal to or double the initial frequency of the alternating current supply, depending, of course, upon whether a half wave or full wave rectifier is employed. In the embodiment shown in the drawing, a full wave rectifier has been employed, and it may be supposed that the impressed current is of the usual sixty cycle alternating character. This voltage is fed through the primary of the transformer 5 to the set proper, and induces a voltage in the secondary of the transformer 5 which is effective across the voice coil 5a. The voltage is also eifective'across the field winding 6, and this winding in the loud speaker also induces a voltage in thevvoice coil 5a, due to the inherent unsteady'fluxjacross the voice .coil. The alternating voltage impressed upon the voice coil 5a in the ways described is considerable, and may be equivalent to the voltage supplied across the primary of the trans former 5. It may be, and usuallyis, of a value of fifteen to twenty-five volts or more. If this were reproduced by the movingsystem of the speaker, it would be unbearable, and not only would produce an extremely disagreeable hum, but in a number of instances would be suflicient to override the desirable signals- By this I [mean that the frequency of one hundred and twenty cycles and the inherent harmonics thereof induced in the voice coil would, many instances,

drown out the signal received from the broadcast station.

In the practice of my invention I offset or balance this hum voltage, and I do it in such a way that a voltage of the same frequency is supplied the output circuit through the grid circuit of the output stage, so that the voltage in the plate cir-' cuit of the output tube (which will clearly be impressed across the primary of transformer 5) will be exactly equal to the said hum voltage directly impressed by the rectifier and one hundredand eighty degrees out of phase therewith.

This is accomplished mainly through the units directly in the grid circuit of the ouput stage. It will be seen in Figure 1 that there are in this circuit the grid load unit 15,- the hum balance resistor 13, a portion of the inductance 6, and.

the filament which I have-designated as 14, connected to ground through resistance 14a. It will.

be noted that the groundlead G is tapped into the inductance 6, dividing it into two effective;

8 is an audio by-pass con- 7 unit 3, and its value may be controlled by the 10 is a radio frequency 7 common to the rectifier circuit. This voltage is,

of course, impressed upon the grid of the output values of the grid return condenser 12 and the hum balance resistor 13 which effectively form a shunt across the tuned and resonant portion of the inductance 6. In practice the condenser 12 will usually be fixed, and will be of such value as to allow a good frequency range for the amplification of the broadcast signals being received and amplified in the set.

The other section of the inductance 6, shunted by the condenser 8 as described, will be the larger section, and also will be of such high resistance that it has substantially no effect upon theoscil latory circuit comprising the condenser 11 and the other section of the inductance 6, regardless of the close nature of the coupling involved. f I have thus described means whereby the hum, which would otherwise be reproduced in the loud speaker by reason of pulsating currents impressed and 'induced in the windings thereof, may be counterbalanced and rendered audibly ineffective by an equivalent voltageoutof phase one hundred and eighty degrees therewith, also pulsat-; ing', applied to the speaker throughthe plate circuit of the output tube and controlled bya pulsating frequency in the grid circuit of the said tube.

'If' the induced. and impressed frequencies referred to were the only elements producing hum in the loud speaker the structure just described wouldbe 'sui'licient. However, a certain amount of hum voltage is developed in the radio frequency end and in the detector, and is of a varying nature, depending in considerable measure upon the particular tube or type of tubes employed. To counterbalance this additional hum voltage, another counterbalancing voltage of fair constancy is introduced into the plate circuits of the radio frequency amplifier and detector units through the series element 9, which is a limiting resistor for the B voltage of the radio and detector tubes 1 5 and the shunt element 10 which is a'capacity be tween the resistance 9 and the ground lead G. The elements are so controlled-that this voltage will be. of suflicient magnitude to be little affected by variations in the tubes. It will be noted that 3 while this voltage is fed into the grid circuits of the radio and detector tubes, it is also in part fed into the grid circuit of the audio tube or tubes. However, I compensate for this by an adjustment of the resistance unit 13. 3

Turning now to Figure 2, I have shown essen-I tially the same elements arranged in connection with an output tube again indicated at 3. In this embodiment, however, the inductance 6 is a not tapped and is only in series with the ngative supply lead of the rectifier 4. The entire coil 6 is shunted by the condenser 11a which, in this instance, I have shown as a variable condenser," so that the proper resonance may be secured for impressing back upon the grid of tube 3 a one hundred and eighty degree out of phase current to produce through the plate circuit of said-tube and transformer 5, counterbalancing voltage upon the voice coils 5a. In-this instance I prefer also to make the resistance element 13 variable, and

I have not shown means for impressing a counterbalancing one hundred and eighty degree out of phase voltage on the grid circuits of radio and detector tubes. Such a feature may, of course, be added if desired. The arrangement shown in Figure 2 is especially applicable where a low excitation current is required by the speaker, and under these conditions operates practically as well as the embodiment shown in Figure 1, although in general the latter embodiment is to be preferred.

It will thusbe seen that I have not only directly excited my loud speaker field circuit by pulsating direct current, but I have used a pulsating I direct current for the plate circuits of my tubes and for the grid bias thereof, and I have counterbalanced the hum forming voltage not only by producing in the plate circuit of the output tube an equivalent voltage which is one hundred and eighty degrees out of phase, but also by counterbalancing a hum voltage transmitted into the circuits of the output tube by preceding stages of the set. It will be seen that my invention possesses a number of distinct advantages over the brute force filter commonly used, and is distinctly economical, since the condenser 11 is the only part necessary over what would be required to operate the set entirely without any filtering at all. The amount of straight filtering in my system is obviously very little, and the condenser 11, instead of being of a size comparable to the filtering condensers ordinarily used in radio sets, need not be more than 0.5 microfarads, ordinarily.

.; Again, the rectifier system may be operated at "than can be attained by any but the most expensive and complicated filtering systems, the cost of which has become prohibitive in the great majority of radio sets being produced today.

Modifications may be made in my invention without departing from the spirit thereof.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is:-

1. In a radio set a dynamic speaker having a field winding and a voice coil, means for impressing a rectified alternating, and consequently pulsating current directly upon said field coil and upon the plate circuit of an output stage in said set, and means for impressing upon said speaker through said output stage an equivalent one hundred and eighty degree out of phase voltage, whereby the tendency to hum reproduction is neutralized, said means comprising a condenser shunted across at least a portion of said field winding to produce an oscillatory circuit tuned to the frequency of said impressed pulsating current, and a connection between said oscillatory circuit and the grid of a tube in the output stage of said set.

2. In a radio set a dynamic speaker having a field winding and a voice coil, means for impressing a rectified alternating, and consequently pulsating current directly upon said field coil and upon the plate circuit of an output stage in said set, and means for impressing upon said speaker through said output stage an-equivalent one hundred and eighty degree out of phase voltage, whereby the tendency to hum reproduction is neutralized, said means comprising a condenser shunted across at least a portion of said field winding to produce an oscillatory circuit tuned to the frequency of saidimpressed pulsating current, and a connection between said oscillatory circuit and the grid of a tube in the output stage of said set, said connection containing a controlling resistance.

3. In a radio set a dynamic speaker having a field winding and a voice coil, means for impressing a rectified alternating, and consequently pulsating current directly upon said field coil and upon the plate circuit of an output stage in said set, and means for impressing upon said speaker th ough said output stage an equivalent one hundred and eighty degree out of phase voltage, whereby the tendency to hum reproduction is neutralized, said means comprising a condenser shunted across at least a portion of said field winding to produce an oscillatory circuit tuned to the frequency of said impressed pulsating current, a connection between said oscillatory circuit and the grid of a tube in the output stage of said set, and a shunt connection between the said resonant circuit and ground comprising a capacity and a resistance.

4. In a radio set a dynamic speaker having a 105 field winding and a voice coil, means for impressing a rectified alternating, and consequently pulsating current directly upon said field coil and upon the plate circuit of an output stage in said set, and means for impressing upon said speaker 110 through said output stage an equivalent one hundred and eighty degree out of phase voltage, whereby the tendency to hum reproduction is neutralized, said means comprising a condenser shunted across at least a portion of said field 115 winding to produce an oscillatory circuit tuned to the frequency of said impressed pulsating current, a connection between said oscillatory circuit and the grid of a tube in the output stage of said set, and means for impressing a one hundred 120 and eighty degree out of phase component of said impressed current upon the grids of tubes in other than the output stage of said set.

5. In a radio set a dynamic speaker having a field winding and a voice coil, means for impressing a rectified alternating, and consequently pulsating current directly upon said field coil and upon the plate circuit of an output stage in said set, and means for impressing upon said speaker through said output stage an equivalent one hundred and eighty degree out of phase voltage, whereby the tendency to hum reproduction is neutralized, said means comprising a condenser shunted across at least a portion of said field winding to produce an oscillatory circuit tuned to the frequency of said impressed pulsating current, a connection between said oscillatory circuit and the grid of a tube in the output stage of said set, and means for impressing a one hundred and eighty degree out of phase component of said 140 impressed current upon the grids of tubes in other than the output stage of said set, said means comprising a resistor and a capacity shunted between said connection and ground.

6. In a radio set a dynamic loud speaker having a tapped field winding, means for impressing a rectified alternating, and consequently pulsating unidirectional current upon said field winding, condensers respectively shunting the portions of said winding divided by said tap, one of said 150;-

condensers and 'a portion of said winding shunted thereby forming an oscillatory circuit tuned to the frequency of said pulsating current, a connection between-said oscillatory circuit and the grid lead of a tube in the outputstage, a transformer the primary of which is in series with said field Winding and the plate of a tube in said output stage, the secondary of which is connected with the voice coil of said loud speaker, and control means for said severalcircuits whereby a pulsating current is directly impressed on said voice coil, and a compensating current one hundred and eighty degrees outof phase therewith is also-impressedtherecn, the frequencies of said counterbalancing currents neutralizing each other.

7. In a radio set having aloud speaker and an output transformer, said loud speaker having a voice coil and a field winding, means for supplying said set with alternating current for operation without audibly reproducing the frequencies thereof, which comprises means for rectifying said alternating current and for impressing a' portion thereof directly upon the field Winding of said loud speaker, and means for impressing a portion thereof upon the voice coil of said speaker through said transformer, whereby a pulsating voltage is .effectiveuponthe voice coil of said speaker, and

effectively counter-balanced, and means for additionally impressing upon the voice coil of said speaker afivoltage'sufiicient to counter-balance a hum voltage impressed upon said speaker by stages of said set other than said output stages.

' EMMERY J H, BUSSARD. 

